Most preferably, the invention relates to pharmaceutical preparations for topical application which contain a compound of the formula I, wherein R1 and R2 are as defined above and one of R3, R4 and R5 is hydrogen and the others are lower alkyl containing up to and including 4 carbon atoms, such as ethyl, or 4- to 7-membered 3-oxa-lower alkylene such as 3-oxa-1,5-pentylene, and to the use of these